`tag.destroy` will never execute since `taggings_count` was cached in memory.

* ruby (2.1.2)
* rails (4.1.1)
* acts-as-taggable-on (3.2.6)

This commit fix the bug and add a test as well.
Merge pull request #560 from tonytonyjan/patch-1

Fix `ActsAsTaggableOn.remove_unused_tags`